Problem: Is ${526443}$ divisible by $3$ ?
Solution: A number is divisible by $3$ if the sum of its digits is divisible by $3$ . [ Why? First, we can break the number up by place value: $ \begin{eqnarray} {526443}= &&{5}\cdot100000+ \\&&{2}\cdot10000+ \\&&{6}\cdot1000+ \\&&{4}\cdot100+ \\&&{4}\cdot10+ \\&&{3}\cdot1 \end{eqnarray} $ Next, we can rewrite each of the place values as $1$ plus a bunch of $9$ s: $ \begin{eqnarray} {526443}= &&{5}(99999+1)+ \\&&{2}(9999+1)+ \\&&{6}(999+1)+ \\&&{4}(99+1)+ \\&&{4}(9+1)+ \\&&{3} \end{eqnarray} $ Now if we distribute and rearrange, we get this: $ \begin{eqnarray} {526443}= &&\gray{5\cdot99999}+ \\&&\gray{2\cdot9999}+ \\&&\gray{6\cdot999}+ \\&&\gray{4\cdot99}+ \\&&\gray{4\cdot9}+ \\&& {5}+{2}+{6}+{4}+{4}+{3} \end{eqnarray} $ Any number consisting only of $9$ s is a multiple of $3$ , so the first five terms must all be multiples of $3$ That means that to figure out whether the original number is divisible by $3 $ , all we need to do is add up the digits and see if the sum is divisible by $3$ . In other words, ${526443}$ is divisible by $3$ if ${ 5}+{2}+{6}+{4}+{4}+{3}$ is divisible by $3$ Add the digits of ${526443}$ $ {5}+{2}+{6}+{4}+{4}+{3} = {24} $ If ${24}$ is divisible by $3$ , then ${526443}$ must also be divisible by $3$ ${24}$ is divisible by $3$, therefore ${526443}$ must also be divisible by $3$.